A continuous casting process produces a slab by continuously supplying molten steel from a ladle to a mold while temporarily storing the molten steel in a tundish of a continuous caster and cooling the mold.
FIG. 1 is a cross-sectional view illustrating examples of incorrect positioning of a submerged entry nozzle that is inserted down into a mold, and FIG. 2 shows graphs illustrating the result of a numerical analysis of normal or abnormal channelling phenomenon of molten steel according to the positioning of a submerged entry nozzle.
According to the figures, a submerged entry nozzle 5 that supplies molten steel into a mold 3 is mounted under the tundish 1. The submerged entry nozzle 5 is mounted to penetrate and extend out from the bottom of the tundish 1 while passing through a well block 7, which is inserted through the bottom of the tundish 1, and a nozzle connector 9 attached to the bottom. The nozzle connector 9 holds the upper portion of the submerged entry nozzle 5 to maintain the perpendicularity of the submerged entry nozzle.
Two opposing molten steel discharge holes 11 are formed at a lower portion of the submerged entry nozzle 5. A stopper 13 is disposed over the submerged entry nozzle 5, which controls the amount of supply of molten steel into the mold 3 by opening/closing the submerged entry nozzle 5.
Installation of the tundish 1 having this configuration is completed by inserting the submerged nozzle 5 down into the mold 3 and positioning the lower end of the submerged entry nozzle 5 inside the mold 3, in which a centering operation of the submerged entry nozzle 5 is performed.
The centering operation of the submerged entry nozzle 5 is performed by moving the tundish 1, using a cylinder, in which the quality of a product is largely influenced by design factors, such as the shape, size, and position (submerged depth) of a molten steel discharge hole, and operational factors, such as the initial installation position before casting starts and a position change generated in casting.
That is, when the submerged entry nozzle 5 is accurately centered, as shown in FIG. 2A, a left-right symmetric flow pattern is formed in the long side (defining the width of billet) and short side (defining the thickness of billet) directions of the mold 3 and consistent initial solidification is ensured, such that it is possible to manufacture a fine billet or a defect-free billet.
However, thermal deformation in the longitudinal direction of the mold 3 is easily generated when the tundish 1 is used over a long period of time, and as a result, the tundish 1 becomes eccentric at one side in the longitudinal direction. The eccentricity of the tundish 1 reduces the accuracy for centering the submerged entry nozzle 5 in the mold 3, even if the submerged entry nozzle 5 is installed perpendicular at the lower portion of the tundish 1.
As described above, when casting is performed by opening the stopper 13, with the submerged entry nozzle 5 inaccurately centered, as shown in FIGS. 2B and 2C, the molten steel becomes concentrated at one side within the mold 3, thereby generating a channelling phenomenon of the molten steel.
The channelling of the molten steel is closely related to the error in the installation position of the submerged entry nozzle 5, and the installation position error of the submerged entry nozzle 5 is caused mainly by off-centering that occurs in actual continuous casting. Examples of the off-centering are as follows.
There are some cases, such as, first, when the center of the submerged entry nozzle 5 deviates from the center of the mold 3 to the left direction of FIG. 1A, second, when it deviates from the center to the right direction of FIG. 1B, and third, when the submerged entry nozzle 5 is tilted at an angle as in FIG. 1C, which may be caused by inaccurate connection of the nozzle connector 3 or its rotation during changing the submerged entry nozzles 5.
The channelling of the molten steel S inside the mold by the off-centering causes an increase in the fluctuation of the surface of the molten steel or vortex, such that mold powder becomes entrapped in the molten steel S, thereby causing non-uniform solidification. The non-uniform solidification generates non-uniform solidified shells and deteriorates the quality of a slab, and if excessive, it causes a break-out in which the billet explodes and the molten steel flows out during casting.
The break-out increases the possibility of a safety accident to workers and damage to the equipment. Further, as the equipment is damaged, the entire operation should be stopped and the operation equipment should be reset, thereby reducing manufacturing efficiency.